Heat exchangers are devices used to transfer heat between a hot fluid stream and a cold fluid stream. In conventional heat exchangers the heat is transferred from one stream to another through a wall and the heat transfer is limited by the conductivity of the material of which the wall is made.
Regenerative heat exchangers typically are capable of achieving higher heating temperatures. Regenerative heat exchangers expose a heat-absorbing mass or matrix alternately to a hot stream and to a cold stream. In general, therefore, regenerative heat exchangers have periodic flow.
Periodic-flow exchangers operate differently from conventional fixed-surface heat exchangers in that heat is transferred from the hot fluid to the cold fluid by alternatively heating and cooling a high surface area matrix material. This matrix or core is either rotated through or shuttled back and forth between the hot and cold fluid streams of the fluid streams are switched between or among two or more stationary matrices. One type of periodic flow regenerative heat exchanger is the rotary regenerative heat exchanger in which a heat-absorbing matrix is rotated relative to streams of hot and cold fluids. The matrix generally comprises a disk or drum-shaped body having a plurality of internal passageways oriented axially. The fluid streams flow through these passageways alternately heating the matrix body or extracting heat therefrom. Such rotary heat exchangers are particularly useful as air preheaters in boiler plants and in gas turbine engines. Seals are provided that either have rubbing contact or maintain a very small gap with the matrix and serve to separate the hot and cold streams thereby reducing leakage losses that occur between the hot and cold fluid streams.
Rotary regenerators have advantages that make them well suited for gas-turbine engines. One of these advantages is compactness. In laminar flow of the fluid streams, the volume needed for a given quantity of heat to be transferred is proportional to the square of the hydraulic diameter of the passage used (Wilson, The Design of High Efficiency Turbomachinery and Gas Turbines, MIT Press, Cambridge, Mass., 1984). The passages in rotary regenerators for gas-turbine applications can be made much smaller than those of conventional tubular or plate fin type heat exchangers. In tubular or plate fin type heat exchangers, problems can be encountered if the passages are small because deposits from the hot and cold fluids can accumulate and block the small passages. This problem is alleviated or reduced in rotary regenerators because the fluid streams alternate and reverse flow direction in each passage, thereby removing deposits and reducing blockage. In addition, because hot and cold-stream separation is controlled by the seals rather than by complex ducts that are required in recuperators, the cost of making many small passages is low.
Another desirable feature of rotary and other regenerators is low pressure drop. The pumping power required to force gas through a heat exchanger is directly proportional to the square of the Mach number and is rather independent of matrix geometry (Wilson, 1984, cited supra. Therefore, large face areas must be used to minimize fluid velocity. In the rotary and other regenerators, elaborate manifold schemes to interleave the fluids are unnecessary, so a large flow area is practical. In contrast, with fixed surface heat exchangers, achieving both compactness and large, interleaved flow areas simultaneously is more difficult.
A problem encountered with conventional rotary regenerators is leakage of fluid from the exchanger which decreases its efficiency. Leakage occurs either through the seals that separate the high and low-pressure chambers or through void-volume carryover. Void volume carryover occurs because hot high pressure fluid trapped in the matrix is carried through the seals during rotation of the matrix to the cool, low pressure side. This leakage, although relatively small, worsens as the speed of rotation of the matrix increases.